It was male semi-salic primogeniture. Philippa was the daughter of the second son, whilst John of Gaunt and his Lancastrians are the fourth son, so an adult Holland married to Philippa, or the adult son of Philippa, would have suceeded peacefully.
There is no guarantee it would be peaceful IMHO. If the Lancastrians were not supporters of the Holland/Clarence line it seems plausible they would oppose their succession. Obviously if Edward IV rules well and unites his nobility his wishes would be accepted but even then the House of Lancaster may be seen as the alternative (much like Mortimer/York was to Lancaster during the reigns of Henry's IV, V and VI) and could cause trouble down the line.